MAP-BASED GENERALIZATION VERSUS STIMULUS-OUTCOME ASSOCIATIONS IN DECISION-MAKING: DISSOCIATING THE CAUSAL CONTRIBUTIONS OF HIPPOCAMPUS AND STRIATUM

In many decision-making situations, humans employ a mixture of two strategies: choosing options rewarded in the past (stimulus-outcome association-based strategy) and generalizing to previously unseen options, with the degree of generalization depending on the proximity between unseen and rewarded options in the individual’s mental representation (cognitive map-based generalization strategy).
Previous research suggests that cognitive maps underlying the latter strategy may be stored in the hippocampal formation, whereas striatum is known to be the site of reinforcement learning mechanisms likely involved in the former strategy.
Non-invasive brain stimulation (NIBS) is a powerful tool for confirming causal brain-behavior relationships. However, the focality of established NIBS techniques is limited for deeper target regions such as hippocampus and striatum. This depth-focality tradeoff has recently been overcome by transcranial temporal interference stimulation (tTIS).
Using tTIS, we aim to isolate the causal contributions of hippocampus versus striatum to map-based versus association-based decision-making. Our participants perform a computerized decision-making task permitting the use of both strategies across three sessions: tTIS enhancing hippocampal activity, striatal activity, and high-frequency control stimulation (order randomized). Per participant and session, a two-component computational model is fit to the choice data to quantify the relative use of either strategy. We hypothesize to observe a shift towards map-based behavior under hippocampal tTIS and towards association-based behavior under striatal tTIS, as compared to control.
Our results will clarify the causal roles of striatum and hippocampal formation in decision-making and pave the way towards understanding decision-making impairments in conditions affecting these brain regions.